The rapid advances which have been made in semiconductor technology have permitted an increased reliance on computers to solve problems. Particularly as a result of the proliferation of small, inexpensive computers, such as microprocessors, the need for digital communication systems allowing a number of computers and computer peripherals to be interconnected in a computer communications network has become of increasing significance. One approach to this problem is to connect the various computer devices to a single communications link via data transmitters and data receivers. One system based on this approach is disclosed in U.S. Pat. No. 4,063,220 issued to R. M. Metcalfe et al. on Dec. 13, 1977. In that system one transmitter at a time is allowed to transmit data and part of the data transmitted defines the address of the destination receiver. Frequently the computers to be interconnected perform diverse functions such as bulk data transfer or signaling. It is undesirable to deny link access to a first computer used for relatively short duration signaling messages until a second computer has completed a bulk data transfer. Accordingly, some means is needed to provide preemptive access to the link on a priority basis.
To assure reliability a protocol or set of rules is typically imposed which governs communication over such a computer network data link. For example, data may be transmitted in variable length frames with predefined fields, and special characters and error check algorithms may be defined. Such protocols are typically implemented via link controllers. To implement error checking, a frame check sequence is derived and transmitted as part of each frame by a transmitting link controller. A receiving link controller then determines if the received frame check sequence is consistent with the rest of the received frame and generates error signals when appropriate. However, known link controllers derive the frame check sequence only over frames comprising a continuous sequence of bits. With such link controllers, a preemption occurring in the middle of a long frame, for example, results in the entire frame having to be retransmitted when access to the link is regained. Accordingly, a recognized problem in the art is providing preemptive access to a communications link on a priority basis without requiring data retransmission.